Thread: How do you know when your brass is kaput?

That's a good tip. I wonder what causes that thin spot in that specific point.[/QUOTE]

The case neck and body is pressed out against the chamber from firing pressure and cannot move reward. The case head-web is thicker and doesn't/should not expand and so doesn't grip the chamber and is forced reward against the bolt,hence the stretch in that area.
This is why its important when full length sizing to size to your chamber and not over size or it will stretch excessively in this spot reducing case life.
Any good loading manual is full of this and lots of other info